This invention relates to cavity ring-down spectroscopy (CRDS), and in particular to systems and methods for locking a laser and a ring-down cavity for CRDS.
Traditional spectroscopic methods are limited in sensitivity to approximately one part per ten thousand (1:10.sup.4) to one part per hundred thousand (1:10.sup.5). Cavity Ring-Down Spectroscopy (CRDS), a technique first described by O'Keefe and Deacon in an article in Rev. Sci. Instrum.59(12):2544-2551 (1988), allows one to make absorption measurements with sensitivities on the order of one part per ten million (1:10.sup.7) to one part per billion (1:10.sup.9) or higher.
In a conventional CRDS system, the sample (absorbing material) is placed in a high-finesse stable linear optical resonator. The intensity of a light pulse introduced in the resonator decreases in time. For an empty cavity, the intracavity light intensity follows an exponential decay characterized by a ring-down rate that depends only on the reflectivity of the mirrors, the separation between the mirrors, and the speed of light in the cavity. If a sample is placed in the resonator, the ring-down is accelerated. Under suitable conditions, the light decay remains exponential. An absorption spectrum for the sample is obtained by plotting the reciprocal of the ring-down rate versus the wavelength of the incident light.
Conventional pulsed CRDS systems have been faced with a number of challenges. In systems using pulsed lasers, data acquisition rates may be limited by the repetition rate of the pulsed laser source. Moreover, the intensity of the light coupled into and out of the cavity may be small, as a consequence of the relatively low spectral overlap between cavity modes and laser linewidth, as well as a lack of significant light buildup within the cavity.
In U.S. Pat. No. 5,528,040, Lehmann proposed the use of continuous-wave (c.w.) laser sources for CRDS, in particular laser diode (LD) sources. To improve the coupling of light into the ring-down cavity, Lehmann suggested optically locking the diode laser using controlled optical feedback from a reference cavity. The system described by Lehmann was subject to optical feedback from the ring-down cavity into the laser. Such optical feedback may cause instability in the laser operation. Moreover, locking was turned off during measurements, such that each measurement of a ring-down decay required re-locking the laser and cavity. Requiring that locking be turned off between measurements may limit the repetition rates achievable with the system.